Rainbow Electronics MAX9768 - Manual (Page 22)

MAX9768
10W Mono Class D Speaker
Amplifier with Volume Control
22 ______________________________________________________________________________________
Power Supplies
The MAX9768 has different supplies for each portion of
the device, allowing for the optimum combination of
headroom power dissipation and noise immunity. The
speaker amplifiers are powered from PV
DD and can
range from 4.5V to 14V. The remainder of the device is
powered by V
DD . Power supplies are independent of
each other so sequencing is not necessary. Power may
be supplied by separate sources or derived from a sin-
gle higher source using a linear regulator to reduce the
voltage as shown in Figure 10.
Component Selection
Input Filter
An input capacitor, C
IN
, in conjunction with the input
resistor of the MAX9768 forms a highpass filter that
removes the DC bias from an incoming signal. The AC-
coupling capacitor allows the amplifier to automatically
bias the signal to an optimum DC level. Assuming zero
source impedance, the -3dB point of the highpass filter
is given by:
Choose C IN so f -3dB is well below the lowest frequency
of interest. Use capacitors whose dielectrics have low-
voltage coefficients, such as tantalum or aluminum elec-
trolytic. Capacitors with high-voltage coefficients, such
as ceramics, may result in increased distortion at low fre-
quencies.
Other considerations when designing the input filter
include the constraints of the overall system and the
actual frequency band of interest. Although high-fidelity
audio calls for a flat-gain response between 20Hz and
20kHz, portable voice-reproduction devices such as cel-
lular phones and two-way radios need only concentrate
on the frequency range of the spoken human voice (typi-
cally 300Hz to 3.5kHz). In addition, speakers used in
portable devices typically have a poor response below
300Hz. Taking these two factors into consideration, the
input filter may not need to be designed for a 20Hz to
20kHz response, saving both board space and cost due
to the use of smaller capacitors.
BIAS Capacitor
BIAS is the output of the internally generated DC bias
voltage. The BIAS bypass capacitor, C
BIAS , improves
PSRR and THD+N by reducing power supply and other
noise sources at the common-mode bias node. Bypass
BIAS with a 2.2µF capacitor to GND.
Supply Bypassing, Layout, and Grounding
Proper layout and grounding are essential for optimum
performance. Use large traces for the power-supply
inputs and amplifier outputs to minimize losses due to
parasitic trace resistance. Large traces also aid in mov-
ing heat away from the package. Proper grounding
improves audio performance, minimizes crosstalk
between channels, and prevents any switching noise
from coupling into the audio signal. Connect PGND and
GND together at a single point on the PCB. Route all
traces that carry switching transients away from GND
and the traces/components in the audio signal path.
Bypass V DD and PV DD with a 1µF capacitor to PGND.
Place the bypass capacitors as close to the MAX9768
as possible. Place a bulk capacitor between PV
DD and
PGND, if needed.
Use large, low-resistance output traces. Current drawn
from the outputs increase as load impedance decreas-
es. High output trace resistance decreases the power
delivered to the load. Large output, supply, and GND
traces allow more heat to move from the MAX9768 to
the air, decreasing the thermal impedance of the circuit
if possible.
f dB
IN IN
R C
−
=
3
1
2 π
GND
GND
MAX9768
SHDN OUT
3.3V
V DD
PV DD IN
1 μF
MAX1726
12V
1 μF
Figure 10. Using a Linear Regulator to Produce 3.3V from a
12V Power Supply
BOOT+
OUT+
AUDIO
INPUT
MAX9768
C IN
BOOT-
OUT-
IN
FB
R IN
R F
Figure 9. Setting Gain